Biological control (biocontrol) of plant pathogens is becoming an important component of plan disease management. Biocontrol potentially offers answers to many persistent problems in agriculture, including those concerning resource limitations, nonsustainable agricultural systems and over-reliance on pesticides (Cook and Baker, 1983, Am. Phytopathol. Soc. St. Paul, Minn. 539 pp). Biocontrol agents are particularly attractive because they may be able to colonize and protect plant portions inaccessible to conventional chemical pesticides (e.g. plant roots in soil) (Harman and Hadar, 1983, Seed Sci. and Technol. 11:893-906; and Akhmad and Baker, Phytopathology (in press)). Much emphasis is now being placed on the purposeful application of specific biocontrol agents, as opposed to improving the level of naturally occurring biocontrol by environmental modification (Cook and Baker, 1983, supra). Many fungi and other microorganisms have been shown to control various plant pathogens, and a few fungal agents now are being used commercially on a limited scale in Europe to control specific pathogens, including Botrytis cinerea on grapes, Ceratocystis ulmi (Dutch elm disease) on elms, and Chondrostsereum purpureum (silver-leaf disease) on ornamental trees.
The successful use of Trichoderma species as biocontrol agents will be greatly enhanced if improved strains are developed. Five species (species aggregates) of Trichoderma (T. hamatum, T. harzianum. T. koningii. T. polysporum and T. viride) are most important for biocontrol (Cook and Baker, 1983, supra). However, biocontrol capability, as well as numerous other desirable or essential traits, is an attribute of specific strains, rather than of particular species or genera, and is extremely variable among strains. For example, most currently utilized strains are unable to grow at either high or low temperatures favored by some plant pathogenic fungi, only a few are rhizosphere competent (Ahmad and Baker, supra: Chao et al., 1986, Phytopathology 76:60-65), and most control only a narrow range of plant pathogens.
The strains presently available were obtained by selection from naturally occurring variation. Typically, large numbers of wild isolates are screened for their ability to control specific pathogens under controlled conditions. Mutation and further selection of strains has been also employed (Papavizas, 1985, Annu. Rev. Phytopathol, 23:23-54., Ahmad and Baker, 1987, supra).
Genetic recombination potentially is a much more powerful method for developing superior biological control strains than selection or mutation. Strains expressing desirable attributes can be used as parents in crosses with other strains expressing other desirable traits. By so doing, progeny with combinations of traits could be developed. Unfortunately, sexual stages are rare or lacking in most strains of Trichoderma spp., and conventional sexual crosses cannot be used to genetically manipulate these fungi.
Protoplast fusion may provide a means of genetically manipulating strains of Trichoderma spp. by initiating parasexuality (Anne and Peberdy, 1975, Arch. Microbiol. 105:201-205; Fincham et al., 1979. Fungal Genetics. 4th ed. Univ. Calif. Press, Berkeley. 636 pp). In fact, interspecific and even intergeneric crosses may be feasible. Intraspecific crosses have been accomplished in Trichoderma reesei (Gracheck, 1984, Protoplast formation, regeneration, and fusion in Trichoderma, Ph.D. Thesis, Univ. Arkansas.; Toyama et al., 1984, Appl. Environ, Microbiol. 47:363-368). Thus it may be possible to combine desirable traits from various parental strains to produce superior biocontrol strains.